This application claims priority from Korean Patent Application Nos. 10-2004-0091913 and 10-2004-0091912, both filed on Nov. 11, 2004 in the Korean Intellectual Property Office, the disclosures of which are incorporated herein in their entirety by reference.
1. Field of the Invention
Apparatuses consistent with the present invention relate to a variable gain amplifier (VGA), and more particularly, to a VGA that has a stable input impedance matching and a stable noise figure (NF) in spite of a change of an amplification gain.
2. Description of the Related Art
Generally, a VGA is used as a pre-power amplifier in a transmission unit of a wireless communication system and maintain an amplitude of a transmission signal constant. A VGA is also used as a low noise amplifier (LNA) in a reception unit of a wireless communication system and operate in a high gain mode when an amplitude of an input signal is small or in a low gain mode when the amplitude of the input signal is large, to thereby provide an appropriate amplification gain.
Such VGAs are designed in consideration of a signal to noise ratio (SNR), a frequency bandwidth, a low distortion factor, linearity, input/output matching, noise characteristics, and the like.
For example, a noise figure (NF) of a VGA used at a first stage of a reception unit has the greatest influence upon an NF of the entire reception unit. Thus, it is important that a VGA has a stable, low NF.
FIG. 1 is a circuit diagram of a conventional VGA. Referring to FIG. 1, the conventional VGA includes an input matching unit 10 comprised of an inductor L1, a cascode amplification unit 20 for amplifying an input signal and outputting an amplified signal, and an output matching unit 30 comprised of an inductor L2 and a capacitor C2.
The input matching unit 10 achieves matching with an input impedance of the cascode amplification unit 20. The output matching unit 30 performs matching with an output impedance of the cascode amplification unit 20.
The cascode amplification unit 20 comprises a common source amplification unit 21 and a common gate amplification unit 23. The common source amplification unit 21 includes a plurality of NMOS transistors N1 through Ni and a plurality of switches SW1 through SWi. The common gate amplification unit 23 is cascode connected to a common drain of the NMOS transistors N1 through Ni.
Gates of the NMOS transistors N1 through Ni, which constitute the common source amplification unit 21, are connected to an input port In and a first bias source Bias 1 via the switches SW1 through SWi, respectively. A gate of an NMOS transistor Nj, which constitutes the common gate amplification unit 23, is connected to a second bias source Bias 2. In FIG. 1, a first capacitor C1 is used to alternating current (AC) ground the NMOS transistor Nj of the common gate amplification unit 23.
The cascode amplification unit 20, in which the common source amplification unit 21 and the common gate amplification unit 23 are coupled together in a cascode configuration, reduces a Miller effect caused by a parasite capacitance between gates and drains of the transistors N1 through Ni. Accordingly, the cascode amplification unit 20 provides excellent frequency characteristics and is thus frequently used in high frequency amplifiers.
In such a conventional VGA, an amplification gain is determined according to selective on/off operations of the NMOS transistors N1 through Ni of the common source amplification unit 21. In other words, when the NMOS transistors N1 through Ni have different transconductance (gm) values, and the switches SW1 through SWi are selectively turned on/off, a value of current induced to the common drain of the common source amplification unit 21 varies according to which one of the NMOS transistors N1 through Ni having different transconductance values is turned on. Consequently, the amplification gain varies according to which of the switches SW1 through SWi is selectively turned on/off.
For example, it is assumed that the first NMOS transistor Ni among the NMOS transistors N1 through Ni has the greatest transconductance and the i-th NMOS transistor Ni has the smallest transconductance. In this case, if the first switch SW1 is turned on and the rest are turned off, the conventional VGA operates in a high gain mode having the greatest amplification gain. On the other hand, if only the i-th switch SWi is turned on, the conventional VGA operates in a low gain mode having the smallest amplification gain.
However, in the conventional VGA as described above, an input impedance of the common source amplification unit 21 varies according to an amplification gain. More specifically, as the NMOS transistors N1 through Ni of the common source amplification unit 21 are selectively turned on/off to change the amplification gain, the input impedance of the common source amplification unit 21 varies. Hence, in the conventional VGA, the input impedance varies according to an amplification gain, and an NF also varies.
FIGS. 2A to 2D show graphs illustrating a gain, a noise figure, input impedance matching, and output impedance matching of the VGA of FIG. 1. The graphs of FIGS. 2A to 2D show results of simulations on a 0.18 μm CMOS RF MOSFET within a frequency range of 4.7 to 5.3 GHz.
Referring to FIG. 2A, the VGA of FIG. 1 in a high gain mode provides fairly good characteristics. However, as shown in FIGS. 2B and 2C, the VGA of FIG. 1 in a low gain mode provides a very bad noise figure and very bad input impedance matching. At an operating frequency of 5 GHz, the VGA in the low gain mode has a large noise figure of about 10 dB and input impedance matching of about −2 dB, which are worse than those in the high gain mode.
To sum up, in a conventional VGA, an input signal is connected to a common source amplification unit comprised of a plurality of transistors, and an input impedance varies according to which one of the transistors is turned on. Thus, with a change of an amplification gain, the input impedance becomes instable, and a noise figure greatly varies.